1. Field of the Invention
The present invention relates to a method of manufacturing bipolar transistor elements in a semiconductor integrated circuit isolated by a silicon oxide film.
2. Description of the Prior Art
As a method of manufacturing bipolar transistor elements of this type, there has been known the method described in U.S. pat. No. 3,684,125 and illustrated in FIGS. 2a to 2i and discussed hereunder.
As shown in FIG. 2a, on the surface of a semiconductor substrate 1 formed with an n.sup.+-type collector region 2, a p-type base layer 10 is epitaxially grown.
Next, as shown in FIG. 2b, the surface of the grown layer 10 is subsequently thinly oxidized, to form an SiO.sub.2 film 5.
Further, as shown in FIG. 2c, a silicon nitride (Si.sub.3 N.sub.4 ) film 6 is formed thereon.
Then, as shown in FIG. 2d, films 5 and 6 are subjected to selective etching treatments.
Subsequently, as shown in FIG. 2e, one part of the grown layer 10 is removed by etching treatment.
Then, as shown in FIG. 2f, using parts as a mask, the layer 10 is selectively oxidized down to the surface portion of the semiconductor substrate 1, to form an SiO.sub.2 film 7.
Thereafter, as shown in FIG. 2g, a high impurity concentration region 2a for collector ohmic contact is formed.
Then, as shown in FIG. 2h, in order to increase the impurity concentration of the base region to a prescribed value, a p-type impurity is added into the surface of the base region, to thereby form a p.sup.+-type semiconductor layer 10'.
Finally, as shown in FIG. 2i, selective diffusion of emitter region 9 is carried out.
With this method, the p-type epitaxial layer 10 is thick, so that unless the diffusion depth of the emitter 9 is made large to some extent, the effective base width cannot be made small. Also, control thereof, is difficult. For these reasons, the prior-art method has been unsuitable for the manufacture of transistor elements for high speed or high frequency use.
In order to produce a transistor element of small base width or having a shallow base, it has been proposed to form an n-type epitaxially-grown layer on the semiconductor substrate with collector diffused therein, to effect isolation oxidation and, thereafter, to diffuse the base into the surface of the n-type epitaxially-grown layer.
According to this method, however, the silicon oxide film 7 for isolation is so formed that it is thick at its peripheral part as illustrated in detail in FIG. 3a. Since the diffusion of a p.sup.+-type impurity layer 4 for forming the base region is effected by employing the SiO.sub.2 film 7 as a mask, the p.sup.+-type diffusion depth at the peripheral part becomes small. As a result, the end part of the collector 3 - base 4 junction Jc extends beneath a shallow part of the SiO.sub.2 film 7, to bring about a state in which a base electrode 11 and the collector region of the n-type epitaxially-grown layer 3 are prone to be short-circuited. Therefore, the method hitherto attempted has had the problem that the yield is very low.